Threaded fastener

ABSTRACT

A screw having a head and a threaded shank and, in the head, a cruciform shaped recess comprising a central cavity and four grooves radiating from the central cavity at 90° angular spacings, the bases of the grooves lying on a conical (or pyramidal) surface and wherein the included angle at the vertex of such cone (or pyramid) has a value within the range of 40° - 45°, the preferred value being 40°.

This invention relates to externally threaded fasteners (hereinafterreferred to as screws) of the type in which the screw has adriver-engaging recess punched in one end, usually in a head at one endof the shank, the recess being of the "cruciform" type having a centralcavity and four grooves radiating from the cavity and spaced at 90°angles therearound, each groove being adapted to receive a radiallyoutwardly projecting wing on the driver for the screw. The outer or basewall of each groove is part of the surface of a truncated cone orpyramid which is co-axial with the axis of the screw and has its largerdiameter at the outer, or open, end of the recess.

When used hereinafter, the expression "a screw of the type specified"means a screw having the above mentioned features.

In practice, when driving such a screw, the value of the maximum drivingtorque which can be applied is governed by:

(A) THE POSSIBILITY OF TORSIONAL FRACTURE OF THE SHANK;

(B) POSSIBILITY OF REAMING OUT THE RECESS;

(C) TENDENCY FOR THE DRIVER TO "CAM-OUT" OF THE RECESS, THAT ISSELF-DISENGAGEMENT OF THE DRIVER FROM THE RECESS DUE TO REACTION FORCES.

So far as (a) and (b) are concerned these are problems which can beovercome simply by increase in the strength of the material of the screwwhere necessary but (c) is a problem which has always been a major causeof trouble in driving of such screws and is one which can only beeliminated, or substantially reduced, by change in the design of therecess based upon a correct understanding of the mechanics of theprocess of camout. We have discovered from our researches that pastattempts and proposals to deal with the problem have not been successfulbecause they have been based upon a false premise, i.e. amisunderstanding of what actually happens, and what leads to camout ofthe driver, during the actual driving of a screw.

Hitherto, in attempts to improve the performance of screws of the typespecified, it has been assumed that the contact between the driving faceof a driver wing and the associated driving wall of the recess groovehas been a face-to-face contact extending over a definite area ofengagement between driver wing face and groove wall. Thus, proposals toimprove performance by changes in the design (i.e. the geometry) of therecess have started from this premise which we have discovered to be afalse premise.

In the theoretically perfect case, which may be reached or approachedwith a perfectly formed recess and exactly matching driver (when new),there is the possibility of face-to-face engagement, but as explainedlater, in the majority of cases in practice this situation does notprevail.

As is demonstrated in more detail later on herein the true position, inpractice, it is that there is only "point" contact between an edge ofthe driver wing and an edge of the groove wall and it can be shown thatresistance to driver cam-out depends upon achieving maximum frictionalresistance between these two edges. Any change in the geometry of therecess, which starts from this premise, brings into consideration aparameter which, so far as we are aware, has hitherto not beenconsidered by others seeking to improve the cam-out resistance of screwsof the type specified. This is a parameter in the geometry of the recesswhich is known, in practice, as the "main cone angle" and this is theincluded angle of the vertex of the conical or pyramidal surface whichdefines the bases of the radially extending, diametrically opposed,grooves.

We have discovered that the value of the main cone angle plays a verysignificant part in the performance of a screw in relation to resistanceto cam-out. However, when considering variation in the value of the maincone angle there are other important factors to be taken into account.

There are two important commercial factors to be considered. One ofthese has to do with the number of different sizes of driver required tocover the whole range of screw sizes. For many years past, the mostcommonly used screw of the type specified is that which is knownworld-wide under the Registered Trade Mark "Pozidriv." For such knownscrews there are five basic recess sizes (designated 0, 1, 2, 3, 4) andfive basic drivers some for each recess size. The main cone angle is thesame for each size of recess but the the proportions differ. The radialextent of the grooves and width of each groove are greatest in thelongest recess size and gradually get smaller as the recess sizes getsmaller. The same applies to the driver as regards radial extent andwidth of wings. One size of recess can be forged in several differentsizes and types of screws and the depth of penetration of the recesswill vary according to the size and type of screw head (i.e. it will beless in the smaller size than in the larger size). Over the whole rangethere are just over 100 different sizes and types of screw head and itis clear that it would be quite impractical to attempt to provide anequivalent number of different sizes of driver so as to have an exactlymatching driver for each size of recess in each size of screw. Becausethe main cone angle is the same for every size of recess it was realisedthat one size of driver could be used to drive several different sizesand types of screw all having the same size recess provided that withthe recess having the smallest depth of penetration the depth ofengagement of the nose of the driver is sufficient to ensure stabilityof the driver during driving. It was found by experience the whole rangeof screw types and sizes most commonly used in practice could be coveredby five sizes of driver corresponding to the five sizes of recess.

We have discovered that there is a practical lower limit on the extentto which the main cone angle of the recess can be reduced, below whichlimit one size of driver will not be capable of use with the same numberof different screw sizes as is the case at present with the known recessabove referred to. Thus, reduction in the size of the main cone anglemust be governed by the need to preserve this feature of multiplicity ofscrew sizes for each size of driver so as not to bring about acommercially unacceptable increase in the number of drivers required tocover the whole range of screw sizes.

The other factor which is of quite significant commercial and economicimportance is that, if there is any change in the form of the recessthen it should be such that there can be a satisfactory degree ofcompatibility between the new form of recess and the drivers which havebeen used for operating on the said known form of recess. It will beappreciated that this is most desirable, if not essential, in order toavoid the expense of having to provide a complete new set of driverswhen there is any change in the shape of recess. Thus, any new form ofrecess should be compatible with the old form of drivers or at leastthere should be required only a minimum inexpensive modification to makeold drivers compatible with any new form of recess.

The object of the invention is to provide improvement in the recess of ascrew of the type specified which will provide an increase in theperformance characteristics of the screw as regards the abovementionedproblem of cam-out of the driver and will also satisfy the twoabovementioned commercial factors.

According to the invention, in a screw of the type specified the maincone angle of the recess (as herein defined) has a value lying withinthe range 40° to 45°.

The abovementioned known recess (R.T.M. Pozidriv) has a main cone angleof 52°. With the present invention providing such a substantialreduction in the value of the main cone angle, it would not be expectedthat it would be possible to maintain (let alone increase) the cam-outperformance characteristic of the recess and still satisfy the second ofthe two abovementioned commercial factors (i.e. compatibility with theexisting form of driver). We have discovered that, contrary to suchexpectation, the cam-out performance of the improved recess when usedwith the existing known driver, is better than that of the said knownrecess (Pozidriv) when used with such known driver (i.e. the knowncompatible Pozidriv recess/driver combination).

The majority of screws of the type specified have a head of enlargeddiameter relative to the threaded shank of the screw and there must beleft a sufficient amount of metal between the outer boundaries of therecess and the outer surfaces of the head to maintain the requiredstrength in the head so that it can withstand the torque applied duringdriving and so that there is no danger of head cracking during the headforming process. Also there is a limit on the maximum axial depth of therecess because the inner or bottom portion of the recess may enter theupper end portion of the shank adjacent the head and must not be so nearthe junction between the head and the shank of the screw as to give riseto the possibility of weakening this junction such as might lead tofracture between the head and the shank under the driving torqueapplied.

Because the improved recess of this invention has a smaller main coneangle than the said known recess, there has to be some correspondingreduction in the axial depth of the recess so as to avoid weakening thescrew head, particularly at the junction between head and shank.However, this reduction in axial depth of the recess can be used toadvantage to overcome another problem which has been encountered byusers of screws, having the said known recess. In said known recessthere are V-shaped valleys in the walls of the central cavity of therecess at positions intermediate circumferentially adjacent grooves andthe walls of these valleys lie in planes which slope inwardly towardsthe central axis of the recess.

This problem is the tendency for the driver to make incorrect engagementwith the recess. When this happens, instead of the driver wings engagingcorrectly in the grooves of the recess the leading ends of the wingsengage in the V-shaped valleys in between the grooves and proper drivingof the screw is impossible. In mass production techniques using a poweroperated driver to drive a large number of screws one after the otherthe driver may be rotating when offered to the recess and this practicegreatly increases the chance of the aforesaid misengagement takingplace. There is the possibility of damage to the driver or to the recessor both and loss of valuable production time.

However, because of the reduction in axial depth of the recess it ispreferably [though not essential as explained later] to have acorresponding reduction in the axial length of the "nose" of the driver(i.e. the already know driver). This involves only a simple grindingoperation to remove a portion from the extremity of the nose, taken in aplane normal to the axis of the driver and it has the effect ofincreasing the diametrically measured distance between opposed wings ofthe driver at this extremity of the nose. Thus the position can beachieved where this distance is greater than the distance between thewidest parts of two opposed V-shaped valleys when these are present inthe improved recess, or, in terms of the geometry of the improved recesshaving V-shaped valleys the distance measured normally between theinnermost ends of the bases of diametrically opposed grooves is greaterthan the distance measured normally between the wide parts of twodiametrically opposed V-shaped valleys. Thus the driver wings cannot nowmake engagement in the V-shaped valleys and this problem of incorrectdriver engagement is eliminated.

Additionally, the recess of this invention leads to further improvementrelated to the manufacture of the screws with reference to the problemknown in the art as "metal fallaway" and this aspect of the invention isdiscussed in more detail in the following which is a more detaileddescription, given by way of example, of one embodiment of the inventionand illustrated in the accompanying drawings wherein:

FIG. 1 is a plan view of the head of a screw having a recess accordingto the invention,

FIG. 2 is a section on the line 2--2 of FIG. 1

FIG. 3 is a view in side elevation showing the nose of a driver engagedin the recess,

FIG. 4 is a section on the line 4--4 of FIG. 3,

FIG. 5 is a broken perspective view showing the nose of a driver engagedin the recess,

FIGS. 6 and 7 are diagrams to illustrate an advantage of the improvedrecess

FIG. 8 is a side elevation of the nose of a driver.

FIGS. 9, 10 and 11 are diagrams and graphs relating to the aspect ofdriving different sizes of screw with one size of driver.

FIGS. 12, 13, and 14 are graphs of tests performed on screws having saidknown recess and screws having the improved recess.

FIG. 15 and FIG. 16 are side and end elevations respectively of a driverfor use with the recess of FIGS. 1 and 2.

FIG. 17 is a fragmentary side view and FIG. 18 is a corresponding endview of a punch for forming a recess according to FIGS. 1 and 2 in thehead of a screw.

Referring to FIGS. 1 and 2 the recess of the invention is shown appliedto a screw having a countersunk head 10 and shank 11, the recesscomprising a central cavity 12 from which the four grooves 13 radiate ina cruciform fashion. In this embodiment of the invention, in betweenadjacent grooves are the aforesaid V-shaped valleys 14. Each groove hasopposed side walls 15, 16 and a base 17, and assuming a normal righthand thread for the screw, each side wall 16 is known as the drivingwall of the groove, being the wall which is engaged by the co-operatingwing of a driver during the operation of driving the screw.

The bases 17 of the grooves each have slight curvature and lie on aconical surface (see dotted lines in FIG. 2) the vertex angle of thisconical surface being the main cone angle hereinbefore referred to. Inthe preferred embodiment of recess the angle 2θ has a value of 40°.

The side walls 15, 16 may be considered as lying in parallel planesalthough in practice, and as shown exaggerated in FIGS. 1 and 2, theseside walls will incline inwardly and downwardly of the recess by a veryslight amount, this being the normal angle of "draft" provided on thepunch which forms the recess in the head of the screw.

Referring now to FIGS. 3, 4 and 5, the nose of a driver having wings 18is shown engaged in the recess, FIGS. 4 and 5 having been drawn to showthe conditions of engagement between driver and recess when torque isbeing applied to the driver. Because of normal engineering tolerancesthe wings of the driver are not a 100% perfect fit in the grooves of therecess and certain clearances exist between the faces of the wings andcorresponding surfaces of the grooves. These clearances have been shownexaggerated in FIGS. 4 and 5 in order to make them discernible.

In the theoretical consideration, when torque is applied to drive thescrew, the engagement between a driver wing 18 and a driving wall 16 ofa groove, is a face to face engagement, but due to the aforesaidclearances this is not so in practice and the engagement is, as shown,virtually point engagement (see X in FIGS. 4 and 5) between edge 19 of adriver wing and the edge 20 of the groove (FIG. 5). Edge 19 is the edgebetween the side face and end face of the driver wing and edge 20 is theedge between the wall 16 of the groove and upper surface 21 of the head10.

Proposals to increase the cam-out resistance of this type of recesshave, in the past, been based upon a consideration of one flat surfacesliding in contact over another which, as we have now shown, is not thetrue case in practice and we believe that this could be the reason whysuch previous proposals have not met with any significant success.However, once it is appreciated that cam-out resistance depends upon thefriction between said two edges engaging with what is virtually pointcontact, then the matter becomes a consideration of thethree-dimensional geometrical relationship between the directions ofmovement of said two edges. When cam-out starts to take place the pointof contact X moves inwardly along the edge 20 as the edge 19 rises outof the recess and this situation can be equated to movement up aninclined plane (having a very steep angle of inclination) with theresistance to cam-out being equivalent to the horizontal force requiredto push a load up the inclined plane. It can be shown that, as the angleof inclination increases, there is reached a critical value at, andabove which, no movement will take place irrespective of the amount offorce applied. (The value of such critical angle will be dependent uponthe co-efficient of friction at the point of contact between the twoedges).

For a given co-efficient of friction we have found that there is acritical value or "critical angle" at or above which value no cam-outwill take place irrespective of the end load being applied axially tothe driver. One has also to take into account the previously mentionedfactor of metal fall-away which is well understood in the art and needsonly brief explanation. When the punch is forming the recess in thescrew head the metal tends to be wedged outwardly and the side walls ofthe grooves do not conform perfectly to the configuration of the punch.This is especially prevelant with "pan" heads and like heads which arenot confined in a die during forming of the recess and are thereforefree to spread radially.

Fall-away means that the side walls in a groove do not lie in parallelplanes (as shown in the drawings) but diverge from one another and asfall-away has most effect in the region of the central cavity 12 thismeans that the edge 20 of the side wall may have a line of directionextending generally at a small angle to the true plane which it shouldoccupy (in the absence of fall-away). Such line is indicated by dottedline Y in FIG. 4 and the aforesaid angle is known as the "fall-awayangle."

The effect of metal fall-away can be reduced if the main cone angle ofthe recess is reduced, which means a reduction in the fall-away angle.We have found that reduction in the value of this fall-away angle isseen to contribute very significantly to increase in cam-out resistanceand thus, from the view-point of recess design it follows that thesmaller the main cone angle the better the resistance to cam-out.

However because of the necessity to retain the aforementioned commercialfactor of one size of driver being usable with a plurality of differentsizes of screw there is a practical limit to the extent to which themain cone angle can be reduced and we have therefore had to determinewhat is the value of this lower limit for the main cone angle.

For this purpose, reference is now made to FIGS. 9, 10 and 11.

The unbroken lines in columns I, II and III of FIG. 9 represent theoutlines of Pan heads with current "Pozidriv" recesses (i.e. the knownrecess referred to). With regard to these outlines columns I, II and IIIare identical and rows a, b, c and d represent screw gauges 10, 8, 6 and5 respectively to a scale of approximately times 10. These four sizeshave been chosen because they are in very common use and more importantbecause they are driven with the same size of "Pozidriv" (i.e. a Number2 driver). The outline of the improved recess according to the inventionis shown in broken lines.

Also when considering ways to improve a currently used recess there aretwo other design criteria which need to be met. First the envelopingdiameter over the grooves at the top of the head must not be increasedbecause this will introduce the probability of head cracking during thecold forming operation of the recess. These diameters are shown with thesymbols Da, Db, Dc, Dd in column I(a) to (d) respectively.

Secondly with reference to column I(a) as an example, the distancebetween the underhead shank fillet zone 2(a) and the bottom corner ofthe recess 3(a) in the conventional "Pozidriv" must not be reduced (see4(a) in the improved recess) otherwise the strength of the shank to headjunction will be reduced.

The design of an improved recess with reduced main cone angle musttherefore be considered as follows.

Referring to column I(a), through point 1a, which is positioned at thecurrent overall diameter of the wings at the top of the head, a line 8ais drawn at an angle θ_(a) ' to the main central axis of the screw.θ_(a) ' is called the semi cone angle (i.e. one half the main coneangle). An arc of radius Ra (struck from the centre of the underheadfillet radius) is drawn through point 3a which is the bottom corner ofthe current "Pozidriv" recess. The intersection point of this arc andline 8a defines the bottom corner of the new recess and satisfies thecondition that the distance to the underhead fillet zone 2a is the sameas before. Point 4a also defines the diameter at the bottom of the newrecess (shown with the symbol d_(a) ') and this diameter must bemaintained in the head sizes for screw gauges 8, 6 and 5 (shown in rowsb, c and d respectively) in order to enable a single size of driver tobe used.

Turning now to the construction of the new recess for the size 8 screwgauge. Through point 1b which is positioned at the existing diameterD_(b) for the present "Pozidriv" recess, a line is drawn at the angleθ_(b) ' to the main screw axis. Point 4b is positioned on this line atthe diameter d_(b) ' and this again represents the bottom of the newrecess. The construction for the recesses for numbers 6 and 5 screwgauges follow in exactly the same way. In column I(a) to (d) the brokenlines represent the new recesses and it should be noted that θ_(a) ' =θ_(b) ' = θ_(c) ' = θ_(d) ' and d_(a) ' = d_(b) ' = d_(c) ' = d_(d) '.

It will also be seen that the previously stated head/shank strengthrequirements are met because points 4b, 4c and 4d fall outside therespective arcs of radius Rb, Rc and Rd.

The entire procedure has been repeated in column II(a) to (d) with asmaller value for the angle θ i.e. θ_(a) " etc. is smaller than θ_(a) 'etc.

We now compare columns I(d) and II(d) with respect to the resultingdepths of the new recesses in screws of number 5 screw gauge. These areshown with the symbols h' and h" and it is immediately evident that withthe smaller cone angle the depth of recess is reduced, i.e. h" issmaller than h'. A graph can be drawn showing how the depth of therecess in a Number 5 screw gauge screw varies according to the chosenangle θ. Such a graph is reproduced in FIG. 10 and this showsimmediately that with an angle approximately equal to 14° (i.e. maincone angle of 28°) the depth of the recess in a Number 5 screw gaugehead would reach zero.

Therefore this provides a theoretical absolute limit. Thus the reductionin the main cone angle must be arrested well before the value of 28° inorder to ensure that a recess will exist at all in the Number 5 screwgauge head.

In further considering this aspect we next refer to column III and takethe driving engagement between driver wing and groove wall.

The following is based upon consideration of the theoretical case ofthere being face-to-face contact between driven wing and groove wall butthis is quite compatible with the practical case of edge-to-edge contactbecause once the condition has been established where cam out will notoccur (irrespective of the applied torque) then further increase intorque will lead to the area of engagement between driver wing andgroove wall correspondingly increasing from a blank area of edge-to-edgeengagement towards the full area of face-to-face engagement.

Referring to column III(a), the trapexium 1a6a5a3a approximates to thedriving area per groove of the current "Pozidriv" recess. The moment ofthis area about the main screw axis gives an indication of the torquewhich the recess can transmit. In order to compare these moments of areaon screws of different sizes it is necessary to express them as a ratioof the polar moment of the core diameter of the shank or, forconvenience, of the cube of the core diameter. These moment ratios areindicated for current "Pozidriv" in FIG. 11 (on the ordinate of 26°) forall the four screw sizes under consideration. It will be noted that theratio is smallest for the Number 10 screw gauge and it is approximately0.074. We consider that if this ratio of 0.074 is satisfactory for acurrent Number 10 screw gauge "Pozidriv" screw then it should besatisfactory for the improved recess in a Number 5 screw gauge screw.Referring again to column III(a) the trapezium 1a6 a7a4a approximates tothe driving area of the new recess according to the form shown in columnI(a). Similar trapeziums are shown in columns III(b), III(c) and III(d)and with these the ratios of moments of area relative to polar moment ofcore diameter can be computed. Thus for each screw gauge a graph can beplotted showing the dependence of this moment ratio on the semi coneangle θ. Such a set of graphs is shown in FIG. 11 and from it we canseen that with θ equal to 19° the moment ratio for the Number 5 screwgauge is already in the region of that for the current Number 10"Pozidriv" screw gauge. The value of 20° therefore is a little on thesafe side and is thus chosen as the preferred value (i.e. main coneangle 40°) for the improved recess and is also the lower limit of therange of possible values for the main cone angle of the improved recess.

We have found from experience that the improved cam-out performance ofthe recess can still be obtained with a main cone angle which is a fewdegrees in excess of 40° and our experience to date leads us to believethat a value of 45° is the practical upper limit for the value of themain cone angle at which value the cam-out performance, for the worstconditions, is still appreciably better than with the said known recess.By the "worst conditions" we mean the case of a screw head with maximumangle of fall-away and minimum co-efficient of friction between thematerial of the screw and that of the driver. The co-efficient offriction will vary according to the material of the screw and whether ithas any surface finish, such as cadmium plating. In practice, we findthat the lowest co-efficient of friction is about 0.1, which would be acadmium plated screw and for other material it is about 0.15 to 0.2.

As mentioned previously, we have found that, contrary to what one wouldexpect, the cam-out performance of the improved recess, when used withthe said known driver is better than that of the said known recess. Ithad always been thought by those working in this field that the drivermust match the recess and have a main cone angle the same as that of therecess. We have found that this is not so and, although there may beother additional reasons (of which we are not aware) for theabovementioned improved performance, we believe that the main reason isthus:

FIG. 6 is a diagram in which the outline of the said known recess (solidlines) is superimposed on the outline of the improved recess (chaindotted lines). The angle between the lines 22 is the main cone angle andline 23 is the base plane of the improved recess. FIG. 8 is a diagram ofthe nose of the said known driver, the portion shown in chain dottedlines being that portion which is ground away when this driver is to beused with the improved recess. If one now imagines this driver insertedinto the improved recess then (referring to FIG. 6) the driver noze isshown in outline by the solid lines 24 and chain dotted line 23. Becausethe main cone angle of the improved recess is less than that of thedriver the edge faces 25 of the driver wings enter more deeply into thegrooves of the recess than would the edge faces of a driver which hadthe same main cone angle as the recess (taking into account the normalengineering clearances previously referred to). Hence when torque isapplied to the driver the point of engagement between driver wing faceand groove wall (X in FIG. 4) is now at, or closer to, the outermostextremity of the edge of the groove wall (see X' in FIG. 6). Thus thesaid point of contact is at, or nearer to, the position where the effectof metal fall-away is zero or at a minimum and thus the angle offall-away is zero or at a minimum. Hence the conditions for resistanceto cam-out are more favourable than the corresponding case of the saidknown recess and driver combination.

Furthermore, because the position of engagement between driver wing andgroove wall (X') is further from the screw axis the moment arm ofapplied torque with the improved recess and known driver is greater thanin the corresponding combination of said known recess and driver.

A further advantage arising out of the improved recess can be seen withreference to FIG. 7 and further reference to FIG. 6.

As mentioned, the solid lines in FIG. 6 represent the outline of thesaid known recess and the chain dotted lines represent the outline ofthe improved recess. At the mouth, the diameter measured between theouter ends of opposed grooves, has been kept the same in the improvedrecess as in the known recess, to obviate the danger of weakening thehead which might give rise to cracking during the formation of therecess.

In FIG. 7 the cross-hatched portion is a section through the knowndriver which is shaped to conform to said known recess (i.e. a sectionon the dotted line 26 in FIG. 8) with the noze of the driver having beenincorrectly offered to the recess so that the driver wings 25 instead ofbeing engaged in the grooves 27 have become engaged in the widest partof each of the V-shaped valleys 28. However, in the improved recess, seeFIG. 5, because the main cone angle has been decreased, the overallaxial depth of the recess has been reduced and the distance C measuredbetween the bases of diametrically opposite grooves at the inner end ofthe recess is increased by comparison with the corresponding distance Din the said known recess. From this it will follow that a driver adaptedto conform exactly to the improved recess (i.e. same main cone angle),will have driver wings which are axially shorter than the wings of saidknown driver and also the distance between the extreme edges of opposeddriver wings at the end of the driver will correspond to the distance Cbeing greater than the distance D between the extreme tips of thediametrically opposed driver wings in said known driver.

Referring to FIG. 7 it will be observed that the increase in thedistance between the extreme tips of diametrically opposed driver wingsin such form of driver conforming exactly to the improved recess (asshown in chain-dotted lines 29) means that such distance C is nowgreater than the distance D between the widest part of one V-shapedvalley and the widest part of the diagonally opposite valley. This meansthat if the driver is incorrectly offered to the recess in the mannershown in FIG. 7 the driver wings cannot now make any engagement in theV-shaped valleys and thus incorrect engagement of the driver with therecess is positively eliminated.

Even when taking the shortened form of the known driver having itsground-off end on line 30 (FIG. 8) it will be seen that the sameconditions apply and incorrect driver engagement cannot take place withthe improved recess.

There is a still further advantage gained in using the known driver(shortened as mentioned) with the improved recess and this is inapplications where a screw has to be driven in a confined space and itis not possible to get the axis of the driver properly aligned with theaxis of the screw. For example when driving a screw into a workpieceadjacent a corner, it may be necessary to tilt the driver in relation tothe axis of the screw through an angle of up to 6° or 7° because theobstruction provided by the corner does not permit the driver to beaccurately aligned with the screw axis. When using a driver whichexactly conforms to the shape of the recess in the screw it is verydifficult to satisfactorily drive the screw fully home and whatfrequently happens is that as the screw approaches the final positionthe increased resistance causes cam-out of the driver with the resultthat in the majority of cases the screw cannot be driven fully home andany excessive force used in an attempt to drive the screw fully homeusually results in reaming out of the recess or fracture of the screwshank.

However, using the known driver with the improved recess, because themain cone angle of the driver is greater than that of the recess, therewill be an angular clearance of the order of 6° between the bases of thegrooves of the recess and the corresponding faces of the driver wingswhen the driver is applied axially into the recess with the result thatthe driver can be tilted through such angle away from the axis of thescrew and the wings of the driver will still maintain satisfactorydriving engagement with the grooves of the recess. It will be understoodthat, because the side walls of the grooves of the improved recess arenot truly parallel but have a certain amount of "draft" as previouslymentioned then when the driver is tilted in relation to the recess andin the direction of two opposed wings the remaining two opposed wings,which are at right angles to the first said two wings, will also be ableto tilt because of this slight taper or draft provided on the walls ofthe grooves. Tests have proved that with this capability of being ableto tilt the known driver in engagement with the improved form of recessit is possible to satisfactorily drive screws fully home in suchconfined spaces where true axial alignment of driver and screw is notpossible.

We have carried out tests on a number of screws to demonstrate that therecess according to the invention, with a main cone angle of 40°, doesgive consistently better resistance to cam-out than does the said knownrecess. The apparatus used for such tests comprises a die into which thescrew under test is screwed with a small end load of 2 lb. The dieincorporates strain gauges which measure the applied torque at theinstant that cam-out occurs. The results of some of these tests areshown in FIGS. 12, 13, 14.

In order to test the screws under the sort of conditions likely to beencountered in actual practise (e.g. on the shop floor) the screwdriverbit in the test rig was given an angle of 5° misalignment with the axisof the screw being driven and the screw chosen was a No. 6 gauge panhead screw which is a popular size and because of its pan head is likelyto have some of the effects of metal fall-away in its recess. FIG. 12shows the results obtained with tests on No. 6 screws having the saidknown recess and using the known driver bits. The first thing observedis that it was found not possible to drive more than ten screws with onebit before the bit became so worn that it had to be changed, so that forthe eighty screws tested eight separate bits were required.

From past experience a figure of 45 lbs. ins has been adopted as beingthe maximum safe torque which can be applied to the No. 6 screw astorque in excess of this figure brings in the danger of fracture of thescrew shank below the head. Hence, in the tests of FIG. 12 the appliedtorque was stopped at the value of 45 lbs. ins. if cam-out had notpreviously occurred at a lower torque.

From FIG. 12 it can be seen that of the eighty screws tested only sixreached the safe maximum torque without cam-out occurring. For theremainder cam-out occurred at widely varying torques below this figure,some being as low as 2 lbs. ins., and the results showed greatinconsistency in performance between one screw and another.

Turning now to FIG. 13, this shows results obtained with tests on No. 6pan head screws having the improved recess and using the known driverwhich had been shortened by grinding away a portion of the nose ashereinbefore described, and the first thing to be noted is that it waspossible to test over sixty screws with the same driver bit because, forthe majority of the screws the test was stopped when the safe maximumtorque of 45 lbs. ins. had been reached without cam-out taking place,hence the driver suffered no damage from the effect of cam-out. In a fewcases the torque was taken above 45 lbs. ins., without cam-out takingplace.

Some of the screws tested were cadmium plated and these are seen in FIG.13 marked X as the cases where the test ended before the figure of 45lbs ins. torque was reached, but due to failure of the screw head andnot because of cam out. The explanation for this is that the cadmiumplating reduces the frictional resistance between the thread of thescrew and the material into which it being received and also reduces thefrictional resistance below the head of the screw with the result thatgreater tension can be generated in the shank of the cadmium platedscrew than in a similar non plated screw by the same torque. As nocam-out takes place, this increase in tension stress can be sufficientto cause fracture of the shank below the head.

We have also carried out the same tests on No. 6 pan head screws havingthe improved recess but using the known driver in its original form(i.e. not shortened) see FIG. 14 and we found that the results obtainedwere very much the same as those demonstrated by the FIG. 13 tests (i.e.longer driver life and consistent improved resistance to cam-out) exceptthat in the case of cadmium plated screws (those marked X in FIG. 14)cam-out of the driver took place at lower torque rather than screwfailure as in the tests of FIG. 13.

The tests described herein show that a very substantial increase indriver life can be expected from the improved recess and, in addition,there are also advantages appertaining to the tooling (i.e. the punches)which are used to form the improved recess.

In the present commercial version of said known recess, in an attempt toimprove resistance to cam-out, the recess is designed to have thedriving wall of each groove lying in a plane which slopes at a veryslight angle outwardly away from the screw axis and in the directiondownwardly away from the mouth of the recess, or, in simple mechanicalterms the driving wall is "undercut." The opposed wall of each groovehas a similar but inwards angle of taper to put it on a parallel plane.However this has never had any marked effect in improving resistance tocam out, because it is intended to function on the theoretical conceptof face-to-face engagement (which we have shown herein to be not thegeneral case in practice). Also this design of recess was difficult toproduce, the punches for forming the recesses were difficult to make andwere subject to more than the normal amount of wear due to the absenceof the normal positive draft angles on the face of the punch whichformed the driving walls of the grooves.

With the improved recess, the punch can have the desired positive draftangles on all faces, in accordance with good forging practice and thusmuch longer tool life can be obtained than with the case abovementioned.Also the larger surface area of the punch nose and the shallower depthof the recess both contribute towards longer working life for the punch.

Towards the end of the normal life of a punch which is producing theimproved recess, and due to the normal amount of wear on the punch,there will tend to be less displacement of metal in forming the recessand excess metal will be left in the recess mostly in the inner regionsof the recess formed by the nose of the punch (i.e. in the inner regionsthe recess will be slightly undersize as compared with a recess formedat the beginning of the punch life). However this will not affect theperformance of recess produced at the end of the punch life because theabovementioned angular clearance between the improved recess and theknown driver is sufficient to accommodate any excess metal in therecess.

The invention also includes within its scope novel driver for use withthe improved recess, and one embodiment of such novel driver for usewith the recess of FIGS. 1 and 2 is shown in side and end elevation inFIGS. 15 and 16 respectively. The driver has wings 30 and V-section ribs31 for engaging in the groove and V-shaped valleys respectively in therecess, these being formed on the nose of the driver at the end of ashank 32. Shank 32 may be a short shank as for a bit to be used in apower tool or a longer shank with handle at the end for manualoperation. The included angle of the cone (or pyramid) containingsurfaces of the outer faces of wings 30 has a value within the range 40°to 45° and in the preferred embodiment is 40°. This novel form of driverwhich is compatible with the improved recess is preferred for drivingself tapping screws and self drilling/tapping screws where it is verydesirable to have a good fit in the recess to maintain the driver inactual alignment with the recess. Also some users like to be able to"stick" a screw on the end of the driver when offering a screw incramped conditions and here also the novel, compatible, matching driveris preferred. Thus although not essential, this novel form of driver ispreferred in certain cases.

For this reason the calculation of the preferred and minimum main coneangle of 40° (refer to FIG. 9) has been based on the combination of theimproved recess and said novel compatible driver to give thetheoretically perfect case. However we have shown, from the testsmentioned herein, that the improved cam out performance of the recess ofthe invention is still achieved when using the known form of driver(main cone angle of 52°).

Also the invention includes within its scope a novel punch for forming arecess in a screw head, as shown in fragmentary side view in FIG. 17 andin end view of FIG. 18. The punch has a body 40 and cavity 41 to formthe desired shape of head with the projecting punch nose having ribs 42to form the grooves of the recess and V-shaped ribs 43 to form theV-shaped valleys of the recess shown in FIGS. 1 and 2 the included angleof the cone (or pyramid) containing the outer faces of ribs 42 has avalue within the range 40° to 45° and in the preferred embodiment has avalue of 40°.

I claim:
 1. A screw for reducing cam-out of a driver comprising anelongated threaded shank extending along a central longitudinal axis andhaving a head at one end, said head having an outer face with acruciform type recess extending inwardly therefrom along said axis andterminating at a bottom wall, said recess including a central cavityhaving a generally square cross-section from its outer end to its bottomend and which tapers uniformly from a maximum cross-section at saidouter face of the head to a minimum cross-section at said bottom wall,said recess further including four grooves radiating from the centralcavity at 90° angular spacings symmetrically about said axis and eachgroove extending outwardly from a side of the generally squarecross-section of the central cavity and terminating at an outer wall,the outer wall of each groove tapering downwardly and inwardly from saidouter face to said bottom wall, and said outer walls defining a maincone angle of at least 40° and not more than 45°.
 2. The screw accordingto claim 1 in which said generally square central cavity definesV-shaped valleys communicating with the inner ends of said grooves, andthe distance between diametrically opposed valleys is substantially lessthan the distance along the bottom wall of diametrically opposed groovesso that a driver of a size to be cooperatively received by the groovescannot be accidentally inserted into said valleys.
 3. The screwaccording to claim 1 in combination with a driver having a nose whichincludes four radial wings, the nose being of a size to be receivedwithin said recess for driving engagement with said screw.
 4. Thecombination of claim 3 in which the outer surfaces of the radial wingsof said driver are disposed at an angle which is substantially the sameas the main cone angle of the screw.
 5. The combination of claim 3 inwhich the outer surfaces of the radial wings of the driver are disposedat a main cone angle of 52°.